JP2019166451A - Oxidation catalyst, catalyst support structure, production method for oxidation catalyst, and production method for catalyst support structure - Google Patents

Abstract

To provide an oxidation catalyst containing cerium dioxide particles in which the heat resistance is improved.SOLUTION: The oxidation catalyst 2 comprises a cerium dioxide particle 21 and a metal oxide 22. The cerium dioxide particle 21 contains an auxiliary component that is at least one of lanthanum, aluminum and iron. The metal oxide 22 contains iron and manganese and is held on the cerium dioxide particle 21. In the oxidation catalyst 2, the heat resistance can be improved. Further, an improvement in oxidation performance in a filter supporting the oxidation catalyst 2 can be realized.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an oxidation catalyst, a catalyst supporting structure, a method for manufacturing an oxidation catalyst, and a method for manufacturing a catalyst supporting structure.

  Patent Documents 1 and 2 propose cerium dioxide particles having a transition metal oxide containing iron and manganese on the surface or inside thereof. Such cerium dioxide particles are supposed to be used as an oxidation catalyst in DPF (Diesel Particulate Filter) including, for example, DOC (Diesel Oxidation Catalyst) and CSF (Catalyzed Soot Filter).

JP 2017-186220 A JP 2017-185481 A

  By the way, in DPF, it becomes a high temperature state by combustion of the collected soot. Since the cerium dioxide particles in the oxidation catalyst are usually aggregates (aggregated particles) of cerium dioxide fine particles, the cerium dioxide particles may partially sinter at the high temperature state to reduce the specific surface area. . In this case, catalyst performance will fall. Therefore, it is required to improve heat resistance in an oxidation catalyst containing cerium dioxide particles.

  This invention is made | formed in view of the said subject, and aims at improving heat resistance in the oxidation catalyst containing a cerium dioxide particle.

  The oxidation catalyst according to the present invention includes cerium dioxide particles containing an auxiliary component that is at least one of lanthanum, aluminum, and iron, and a metal oxide that is held by the cerium dioxide particles and contains iron and manganese.

  In one preferable form of the present invention, the mass ratio of the auxiliary component is 3 to 45 mass% in terms of oxide with respect to cerium contained in the cerium dioxide particles.

  In another preferred embodiment of the present invention, the mass ratio of the metal oxide is 5 to 40 mass% with respect to the whole of the oxidation catalyst.

  The catalyst support structure according to the present invention includes a cell structure whose interior is partitioned into a plurality of cells by partition walls, and the oxidation catalyst supported by the partition walls.

  Preferred catalyst support structures do not contain noble metals.

  The method for producing an oxidation catalyst according to the present invention includes a step of generating cerium dioxide particles containing an auxiliary component that is at least one of lanthanum, aluminum, and iron, and a metal oxide containing iron and manganese is held in the cerium dioxide particles. A process.

  The method for manufacturing a catalyst-supporting structure according to the present invention includes a preparation step of preparing a cell structure that is internally partitioned into a plurality of cells by partition walls, and an oxidation catalyst manufactured by the oxidation catalyst manufacturing method. And a supporting step for supporting the substrate.

  For example, in the supporting step, a liquid in which the oxidation catalyst is dispersed is applied to the cell structure.

  According to the present invention, heat resistance can be improved in an oxidation catalyst containing cerium dioxide particles. Moreover, the improvement of the oxidation performance in the filter carrying the oxidation catalyst can be realized.

It is a figure which shows the structure of an exhaust gas purification system. It is a figure which shows a catalyst carrying structure. It is sectional drawing which shows a catalyst carrying structure. It is a figure which expands and shows a part of partition. It is a figure which shows an oxidation catalyst. It is a figure which shows the flow of the process which manufactures an oxidation catalyst. It is a figure which shows the flow of the process which manufactures a catalyst support structure.

<Exhaust gas purification system>
FIG. 1 is a diagram showing the configuration of the exhaust gas purification system 8. The exhaust gas purification system 8 purifies exhaust gas discharged from the engine. The exhaust gas purification system 8 includes a DPF (Diesel Particulate Filter) 81, an SCR (Selective Catalytic Reduction) catalytic converter 85, and a urea injector 86. The DPF 81, the urea injector 86, and the SCR catalytic converter 85 are arranged in this order along the direction in which the exhaust gas flows.

  The DPF 81 includes a DOC (Diesel Oxidation Catalyst) 82 and a CSF (Catalyzed Soot Filter) 83. The DOC 82 includes a honeycomb structure that is internally partitioned into a plurality of cells by partition walls, and a noble metal oxidation catalyst supported on the partition walls. The CSF 83 includes a honeycomb structure similar to the above, and a non-noble metal-based oxidation catalyst supported on the partition walls of the honeycomb structure. Details of the structure of the CSF 83 will be described later. The urea injector 86 is provided in the exhaust gas path between the DPF 81 and the SCR catalytic converter 85. The SCR catalytic converter 85 includes a honeycomb structure similar to the above and an SCR catalyst supported on the partition walls of the honeycomb structure.

The exhaust gas discharged from the engine flows into the DOC 82 of the DPF 81. The exhaust gas contains nitric oxide (NO), oxygen (O ₂ ), and nitrogen (N ₂ ), and reactions of the following formulas 1 and 2 occur in the DOC 82. In the reaction of Formula 1, nitrogen dioxide (NO ₂ ) is generated. In addition, SOF (Soluble Organic Fraction) in the following formula 2 is included in PM (particulate matter) in exhaust gas.

2NO + O ₂ = 2NO ₂ (Formula 1)

SOF + O ₂ = CO, CO ₂ , H ₂ O (Formula 2)

In CSF83, carbon (soot) contained in the exhaust gas is collected. Further, the CSF83, the following formula 3 between the soot and NO _2, reaction of formula 4 and formula 5 (combustion reaction) occurs, NO is generated from the NO _2.

C (soot) + 2NO ₂ = CO ₂ + 2NO (Formula 3)

C (soot) + NO ₂ = CO + NO (Formula 4)

C (soot) +1/2 O ₂ + NO ₂ = CO ₂ + NO (Formula 5)

In the urea injector 86, urea is mixed with the exhaust gas discharged from the CSF 83, and the exhaust gas containing ammonia (NH ₃ ) decomposed from urea flows into the SCR catalytic converter 85. In the SCR catalytic converter 85, NOx contained in the exhaust gas is purified by the reactions of the following formulas 6, 7 and 8.

4NO + 4NH ₃ + O ₂ = 4N ₂ + 6H ₂ O (Formula 6)

NO + NO ₂ + 2NH ₃ = 2N ₂ + 3H ₂ O (Formula 7)

6NO ₂ + 8NH ₃ = 7N ₂ + 12H ₂ O (Formula 8)

The reaction of Formula 7 is called a Fast SCR reaction, and the reaction rate is faster than the reactions of Formula 6 and Formula 8. In order to efficiently advance the reaction in the SCR catalytic converter 85 according to Equation 7, it is required that the amount of NO flowing into the SCR catalytic converter 85 and the amount of NO ₂ be 1: 1. On the other hand, in CSF83, as shown in the above-described Expression 3, Expression 4, and Expression 5, a large amount of NO ₂ is consumed by soot combustion, and NO is generated.

Therefore, in the exhaust gas purification system 8 according to the present invention, a catalyst carrying structure that carries an oxidation catalyst described later is provided as a downstream portion of the CSF 83. The catalyst-supporting structure oxidizes part of NO to generate NO ₂ , that is, converts NO into NO ₂ . As a result, the amount of NO and the amount of NO ₂ flowing into the SCR catalytic converter 85 can be made close to 1: 1, and the reaction in the SCR catalytic converter 85 can be advanced efficiently.

<Catalyst carrying structure>
FIG. 2 is a diagram schematically showing the catalyst support structure 1 that supports the oxidation catalyst. The catalyst support structure 1 is a cylindrical member that is long in one direction, and FIG. 2 shows an end face on one side in the longitudinal direction of the catalyst support structure 1. FIG. 3 is a cross-sectional view showing the catalyst support structure 1, and FIG. 3 shows a part of a cross section along the longitudinal direction of the catalyst support structure 1.

  The catalyst carrying structure 1 includes a honeycomb structure 10 and an oxidation catalyst. The honeycomb structure 10 includes a cylindrical outer wall 11 and partition walls 12. The cylindrical outer wall 11 has a cylindrical shape extending in the longitudinal direction. The cross-sectional shape of the cylindrical outer wall 11 perpendicular to the longitudinal direction is, for example, a circle, and may be a polygon or the like. The partition wall 12 is provided inside the cylindrical outer wall 11 and partitions the interior into a plurality of cells 13. The honeycomb structure 10 is a cell structure that is internally partitioned into a plurality of cells 13 by partition walls 12. The cylindrical outer wall 11 and the partition wall 12 are formed of a porous material. The oxidation catalyst is supported in the pores of the porous material. As will be described later, the exhaust gas passes through the pores of the partition wall 12. In order to improve the strength of the catalyst support structure 1, the thickness of the partition wall 12 is, for example, 50 μm (micrometers) or more, preferably 100 μm or more, and more preferably 150 μm or more. In order to reduce the pressure loss in the partition 12, the thickness of the partition 12 is, for example, 500 μm or less, preferably 450 μm or less.

Each cell 13 is a space extending in the longitudinal direction. The cross-sectional shape of the cell 13 perpendicular to the longitudinal direction is, for example, a polygon (triangle, quadrangle, pentagon, hexagon, etc.), and may be a circle or the like. The plurality of cells 13 typically have the same cross-sectional shape. The plurality of cells 13 may include cells 13 having different cross-sectional shapes. In order to improve the oxidation performance, the cell density is, for example, 8 cells / cm ² (square centimeter) or more, and preferably 15 cells / cm ² or more. In order to reduce the pressure loss, the cell density is, for example, 95 cells / cm ² or less, preferably 78 cells / cm ² or less.

  In the catalyst support structure 1 used for the CSF 83, exhaust gas from the DOC 82 flows with one end side of the honeycomb structure 10 in the longitudinal direction as an inlet and the other end side as an outlet. In a predetermined number of cells 13, a sealing portion 14 is provided at an end on the inlet side, and in the remaining cells 13, a sealing portion 14 is provided on an end on the outlet side. Therefore, the exhaust gas flowing into the honeycomb structure 10 moves from the cell 13 whose inlet side is not sealed through the partition wall 12 to the cell 13 whose outlet side is not sealed (see arrow A1 in FIG. 3). At this time, the exhaust gas is oxidized by the oxidation catalyst supported on the partition wall 12. In each of the end portion on the inlet side and the end portion on the outlet side of the honeycomb structure 10, it is preferable that every other sealing portion 14 is provided along the arrangement direction of the cells 13.

FIG. 4 is an enlarged view showing a part of the partition wall 12. The porous material forming the partition wall 12 is provided with a large number of pores 121, and the oxidation catalyst 2 is supported in the pores 121. In FIG. 4, the oxidation catalyst 2 is indicated by a black circle. In order to improve the conversion rate from NO to NO ₂ (hereinafter simply referred to as “NO ₂ conversion rate”) in the catalyst support structure 1, the supported amount of the oxidation catalyst 2 is, for example, 3 g / L (gram per liter). ) Or more, preferably 5 g / L or more, more preferably 8 g / L or more. In order to reduce the pressure loss, the loading amount of the oxidation catalyst 2 is, for example, 50 g / L or less, preferably 45 g / L or less, more preferably 40 g / L or less. The supported amount (g / L) of the oxidation catalyst 2 indicates the amount (g) of the oxidation catalyst 2 supported per unit volume (L) of the honeycomb structure 10. Details of the oxidation catalyst 2 will be described later.

  A preferred example of the porous material forming the partition wall 12 is ceramic. From the viewpoint of strength, heat resistance, corrosion resistance, etc., cordierite, silicon carbide, alumina, mullite, aluminum titanate, silicon nitride, silicon-silicon carbide based composite material, etc. are preferably used. The silicon-silicon carbide based composite material is formed using silicon carbide as an aggregate and metallic silicon as a binder.

  In order to reduce the pressure loss, the open porosity of the porous material (partition wall 12) is, for example, 25% or more, preferably 30% or more, and more preferably 35% or more. From the viewpoint of the strength of the partition wall 12, the open porosity of the porous material is, for example, 70% or less, and preferably 65% or less. The open porosity can be measured by, for example, Archimedes method using pure water as a medium. . The average pore diameter of the porous material is, for example, 5 μm or more, and preferably 8 μm or more. Similar to the open porosity, the larger the average pore diameter, the lower the pressure loss. In order to improve the oxidation performance, the average pore diameter of the porous material is, for example, 40 μm or less, preferably 30 μm or less, and more preferably 25 μm or less. The average pore diameter is measured, for example, by a mercury intrusion method (based on JIS R1655). Depending on the design of the catalyst support structure 1, the sealing portion 14 may be omitted, and the oxidation catalyst 2 may be held in layers on the surface of the cell 13.

<Oxidation catalyst>
FIG. 5 is a diagram schematically showing one particle of the oxidation catalyst 2. The oxidation catalyst 2 includes cerium dioxide (CeO ₂ ) particles 21 and metal oxides 22 held by the cerium dioxide particles 21. Each cerium dioxide particle 21 is an aggregate (aggregated particle) of fine particles of cerium dioxide, for example. In FIG. 5, one cerium dioxide particle 21 which is an aggregated particle is indicated by one circle. The cerium dioxide particles 21 include an auxiliary component that is at least one of lanthanum (La), aluminum (Al), and iron (Fe). The auxiliary component is one or more selected from the group consisting of lanthanum, aluminum and iron. As will be described later, when the cerium dioxide particles 21 contain an auxiliary component, the heat resistance of the oxidation catalyst 2 is improved.

  In order to improve the heat resistance of the oxidation catalyst 2 more reliably, the mass ratio of the auxiliary component is, for example, 3% by mass or more, preferably 5% by mass in terms of oxide with respect to cerium contained in the cerium dioxide particles 21. % Or more, and more preferably 8% by mass or more. In terms of maintaining predetermined performance with cerium dioxide, the mass ratio of the auxiliary component is, for example, 45% by mass or less, preferably 40% by mass or less, and more preferably 35% by mass or less. When the auxiliary component includes a plurality of types of elements, the mass ratio is a total mass ratio of the plurality of types of elements. The mass ratio of the constituent components in the oxidation catalyst 2 can be quantified by, for example, ICP (Inductively Coupled Plasma) emission spectroscopy. In an example of the production of the oxidation catalyst 2 to be described later, when the cerium dioxide particles 21 are generated, the mass ratio can be adjusted by changing the mixing ratio of cerium dioxide and the auxiliary component raw material.

In an example of the cerium dioxide particles 21, fine particles of an auxiliary component oxide (for example, lanthanum oxide (La ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), or iron oxide (Fe ₂ O ₃ )) are formed of cerium dioxide. Aggregates with fine particles. In another example of the cerium dioxide particles 21, the auxiliary component is dissolved in the cerium dioxide crystals. Of course, in the cerium dioxide particles 21, both the oxide fine particles of the auxiliary component and the auxiliary component dissolved in the crystal of cerium dioxide may exist.

  Considering the pore diameter of the porous material, the average particle diameter of the cerium dioxide particles 21 is, for example, 30 μm or less, preferably 20 μm or less, more preferably 10 μm or less. Moreover, the average particle diameter of the cerium dioxide particles 21 is, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 2 μm or more. The average particle diameter of the cerium dioxide particles 21 is calculated by, for example, calculating the average value of the particle diameters of the cerium dioxide particles 21 in an image obtained by photographing the oxidation catalyst 2 at a predetermined magnification using a scanning electron microscope (SEM). Desired. The average particle diameter may be obtained by a laser diffraction method.

The metal oxide 22 contains iron (Fe) and manganese (Mn). In the oxidation catalyst 2, the presence of the metal oxide 22, it is possible to properly oxidize NO contained in the exhaust gas to NO _2. Typically, the metal oxide 22 adheres to the surface of the cerium dioxide particles 21 while being dispersed on the surface. That is, the metal oxide 22 is fine particles attached to the cerium dioxide particles 21. When the cerium dioxide particles 21 are regarded as carriers, the metal oxide 22 is a supported material. In FIG. 5, the iron oxide 22a and the manganese oxide 22b included in the metal oxide 22 are indicated by small circles. The metal oxide 22 may be in a state of covering the surface of the cerium dioxide particles 21. Also, some of the fine particles of the metal oxide 22 may be held inside the cerium dioxide particles 21 (for example, between the fine particles of cerium dioxide). The average particle diameter of the metal oxide 22 is smaller than the average particle diameter of the cerium dioxide particles 21 and is, for example, 0.5 μm or less. The average particle diameter of the metal oxide 22 can be obtained using a scanning electron microscope, similarly to the cerium dioxide particles 21.

An example of the metal oxide 22 is composed only of an oxide containing iron and an oxide containing manganese. In the present embodiment, the metal oxide 22 is at least one of FeMnO ₃ , Fe ₂ O ₃ , and Mn ₂ O ₃ . May be Mn solid solution in Fe ₂ O _3, Fe may be solved in _Mn 2 _{O 3.} Fe ₂ O ₃ and Mn ₂ O ₃ are stable even in a temperature range of 200 to 800 ° C. Depending on the design of the oxidation catalyst 2, the metal oxide 22 may contain other metal elements. Typically, the metal oxide 22 is a transition metal oxide containing only a transition metal.

  In order to exert high catalytic performance by the metal oxide 22, the ratio of the mass of the metal oxide 22 in the oxidation catalyst 2, in this embodiment, the ratio of the total mass of the oxide containing iron and the oxide containing manganese is For example, it is 5% by mass or more, preferably 10% by mass or more, and more preferably 15% by mass or more with respect to the entire oxidation catalyst 2. When the mass ratio of the metal oxide 22 becomes excessively high, the entire surface of the cerium dioxide particles 21 is covered with the metal oxide 22, and the adsorption performance of nitrogen monoxide by the cerium dioxide particles 21 decreases. Therefore, from the viewpoint of securing a certain degree of adsorption performance by the cerium dioxide particles 21, the mass ratio of the metal oxide 22 is, for example, 40% by mass or less, preferably 35% by mass or less, more preferably 30%. It is below mass%.

The mass ratio of manganese in the metal oxide 22 is, for example, 10% by mass or more, and preferably 20% by mass or more in terms of oxide with respect to the total mass of iron and manganese. The mass ratio of manganese is, for example, 90% by mass or less, and preferably 80% by mass or less. In one example of the production of the oxidation catalyst 2 described later, the mass ratio can be adjusted by changing the composition ratio of manganese and iron in the solution used when the cerium dioxide particles 21 hold the metal oxide 22. Furthermore, the crystal structure of the metal oxide 22 can be adjusted by changing the firing temperature. The crystal structure of the metal oxide 22 is, for example, a hematite type (Mn solid solution Fe ₂ O ₃ ) or a bixbite type (Fe solid solution Mn ₂ O ₃ ).

<Method for producing oxidation catalyst>
FIG. 6 is a diagram showing a flow of processing for manufacturing the oxidation catalyst 2. First, the raw material of the auxiliary component is dissolved in water to obtain an aqueous solution of the auxiliary component. The raw materials for the auxiliary components are, for example, lanthanum nitrate (La (NO ₃ ) ₃ .6H ₂ O), aluminum nitrate (Al (NO ₃ ) ₃ · 9H ₂ O), iron nitrate (Fe (NO ₃ ) ₃ · 9H ₂ ). O). Subsequently, cerium dioxide powder is mixed into the aqueous solution. Water is evaporated from the aqueous solution to obtain a powder of a mixture containing the auxiliary component and cerium dioxide. And the said powder is baked in air | atmosphere at predetermined temperature (for example, 500-700 degreeC), The cerium dioxide particle 21 containing an auxiliary component is produced | generated (step S11). In the cerium dioxide particles 21, for example, oxide fine particles of the auxiliary component are aggregated together with the fine particles of cerium dioxide, or the auxiliary component is dissolved in the cerium dioxide crystal.

Subsequently, the cerium dioxide particles 21 are mixed in an aqueous solution in which the raw material of the metal oxide 22 is dissolved in water. The raw material of the metal oxide 22 contains iron and manganese, for example, iron nitrate (Fe (NO ₃ ) ₃ · 9H ₂ O) and manganese nitrate (Mn (NO ₃ ) ₂ · 6H ₂ O). Water is evaporated from the aqueous solution, and a powder of a mixture containing iron, manganese and cerium dioxide particles 21 is obtained. And the metal oxide 22 containing iron and manganese is hold | maintained (supported) by the cerium dioxide particle 21 by baking the said powder in air | atmosphere at predetermined temperature (for example, 500-700 degreeC) (step S12). ). The oxidation catalyst 2 is manufactured by the above process. Typically, the metal oxide 22 adheres to the surface of the cerium dioxide particles 21 while being dispersed on the surface.

<Method for producing catalyst-supporting structure>
Next, production of the catalyst support structure 1 using the oxidation catalyst 2 will be described. FIG. 7 is a diagram showing a flow of processing for manufacturing the catalyst carrying structure 1. First, the honeycomb structure 10 is prepared and prepared (step S21). In the manufacture of the honeycomb structure 10, for example, a formed body is manufactured by extruding a clay containing a ceramic raw material, a binder, a pore former, and the like. The molded body is a cylindrical member, and the inside is partitioned into a plurality of cells by partition walls. One end of each cell is filled with a slurry for forming a sealing portion as necessary. And the honeycomb structure 10 is produced by baking a molded object. Prior to firing the molded body, the molded body may be dried and calcined.

  When the honeycomb structure 10 is prepared, a liquid (slurry) in which the oxidation catalyst 2 is dispersed is applied to the honeycomb structure 10. In one example, the honeycomb structure 10 is immersed in a slurry in which the oxidation catalyst 2 is dispersed in water. The honeycomb structure 10 is dried after being taken out of the slurry. Then, the weight of the honeycomb structure 10 after drying is measured. The application of the slurry to the honeycomb structure 10 and the drying are repeated until the weight of the honeycomb structure 10 after drying increases by a predetermined amount from the pre-measured weight before applying the slurry. Thereafter, the honeycomb structure 10 is fired at a predetermined temperature (for example, 300 ° C.). It is preferable that the said temperature is lower than the calcination temperature at the time of manufacture of the oxidation catalyst 2, and the calcination temperature of the said molded object. In this way, the oxidation catalyst 2 is supported on the partition walls 12 of the honeycomb structure 10 (step S22). In the present embodiment, the oxidation catalyst 2 is supported in the pores 121 of the partition walls 12. The catalyst carrying structure 1 is manufactured by the above processing. Since the preferred catalyst-supporting structure 1 does not contain a noble metal catalyst, it can be manufactured at low cost.

<Comparison with Comparative Example of Catalyst Support Structure>
Here, the catalyst support structure in which the cerium dioxide particles of the oxidation catalyst do not contain an auxiliary component is compared with the catalyst support structure 1 as a catalyst support structure of a comparative example. The catalyst support structure of the comparative example is the same as the catalyst support structure 1 except that the cerium dioxide particles do not contain an auxiliary component.

  When comparing the catalyst support structure 1 having the same amount of the oxidation catalyst supported with the catalyst support structure of the comparative example, the catalyst support structure 1 has a pressure loss (so that a constant flow rate) when no soot is deposited. This is a pressure loss with respect to gas and is also called an initial pressure loss.) Is smaller than that of the catalyst support structure of the comparative example. The reason why the pressure loss is small is not clear, but when the cross section of these catalyst supporting structures is observed with a scanning electron microscope, a certain amount of the oxidation catalyst is solidified in the catalyst supporting structure of the comparative example ( There are some parts that are agglomerated) and attached to the partition walls. On the other hand, in the catalyst supporting structure 1, the degree of dispersion of the oxidation catalyst 2 is higher than that of the catalyst supporting structure of the comparative example. Therefore, in the catalyst-supporting structure 1, it is considered that the fact that the oxidation catalyst 2 that is present in a solid state (that is, the oxidation catalyst 2 that substantially closes the pores 121) is small is one factor that reduces the pressure loss.

  The reason why the degree of dispersion of the oxidation catalyst 2 in the catalyst supporting structure 1 is high is that when the oxidation catalyst is supported on the honeycomb structure, the dispersion state of the oxidation catalyst 2 in water is different from the oxidation catalyst of the comparative example. It is possible. Specifically, the oxidation catalyst of the comparative example tends to agglomerate in water, whereas the oxidation catalyst 2 has an auxiliary component in the cerium dioxide particles 21, so that the surface potential in water is that of the oxidation catalyst of the comparative example. Unlike the surface potential, it is presumed that the oxidation catalyst 2 is easily dispersed in water.

When the catalyst supporting structure is used for the CSF 83 in the exhaust gas purification system 8 of FIG. 1, the conversion rate from NO to NO ₂ (that is, the NO ₂ conversion rate) is improved by increasing the supported amount of the oxidation catalyst. Is possible. On the other hand, when the amount of the oxidation catalyst supported is increased, the initial pressure loss also increases. In the exhaust gas purification system 8, the maximum allowable initial pressure loss is set, and in the oxidation catalyst 2 in which the pressure loss is low, the supported amount in the catalyst supporting structure can be increased as compared with the oxidation catalyst of the comparative example. It becomes possible. As a result, the oxidation performance of the catalyst-carrying structure 1 that is a filter carrying the oxidation catalyst 2 can be improved.

Actually, in the catalyst-supporting structure 1, the contact area of the oxidation catalyst 2 with the exhaust gas increases due to the high degree of dispersion of the oxidation catalyst 2. Further, it is considered that the activity of the metal oxide 22 is increased due to the influence of the auxiliary component. Therefore, in the catalyst-supporting structure 1 in which the cerium dioxide particles 21 include the auxiliary component, it is expected that the NO ₂ conversion rate is increased also from this viewpoint. In the catalyst support structure 1, oxidation of SOF (soluble organic component) such as hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gas may be performed.

In addition, as described above, the CSF 83 becomes a high temperature state by the combustion of the collected soot. When the heat treatment assuming the high temperature state (for example, heat treatment at 750 ° C.) is performed on both the catalyst support structure of the comparative example and the catalyst support structure 1, the catalyst support structure of the comparative example The cerium dioxide particles that are agglomerated particles are partially sintered, and the specific surface area of the oxidation catalyst is greatly reduced. As a result, the contact area between the oxidation catalyst and the exhaust gas decreases, and the NO ₂ conversion rate after the heat treatment becomes low in the catalyst support structure of the comparative example.

On the other hand, in the catalyst-supporting structure 1, the cerium dioxide particles 21 contain an auxiliary component, so that sintering is suppressed and a decrease in the specific surface area of the oxidation catalyst 2 can be suppressed. As a result, the deterioration rate of the NO ₂ conversion rate before and after the heat treatment is reduced, and the NO ₂ conversion rate after the heat treatment is higher than that of the catalyst support structure of the comparative example in which the amount of the oxidation catalyst supported is the same. Thus, in the oxidation catalyst 2, it is possible to suppress a decrease in catalyst performance due to heat treatment, that is, to improve heat resistance. As described above, in the oxidation catalyst 2, the amount of the catalyst supported structure can be increased as compared with the oxidation catalyst of the comparative example. Therefore, coupled with the suppression of the decrease in the specific surface area, the oxidation performance of the filter can be greatly improved. For example, the BET method can be used for measuring the specific surface area.

<Example>
Next, examples will be described. Here, as Examples 1 to 11 and Comparative Examples 1 to 3, an oxidation catalyst and a catalyst-supporting structure were produced under the conditions shown in Table 1.

(Examples 1 to 11)
In the preparation of the oxidation catalyst, first, powder of auxiliary nitrate (La (NO ₃ ) ₃ .6H ₂ O, Al (NO ₃ ) ₃ · 9H ₂ O, Fe (NO ₃ ) ₃ · 9H ₂ O) and water Were weighed and nitrate was dissolved in water in the container to obtain an aqueous solution containing auxiliary components. In Examples 1 to 6 and 9 to 11, lanthanum (La) is used as an auxiliary component, in Example 7, aluminum (Al) is used as an auxiliary component, and in Example 8, iron (Fe) is used as an auxiliary component. It was.

Next, cerium dioxide (CeO ₂ ) powder was weighed and mixed with the aqueous solution. The mixing ratio (molar ratio) between the auxiliary component and cerium (Ce) is as shown in “La / Ce molar ratio”, “Al / Ce molar ratio” and “Fe / Ce molar ratio” in Table 1. The aqueous solution was stirred at 90 ° C. for about 5 hours using a hot stirrer. After visually confirming that the water in the container had disappeared, the mixture in the container was sufficiently dried at 90 ° C. for about 5 hours in a dryer to obtain a powder containing auxiliary components and cerium dioxide. The powder was fired at 500 to 700 ° C. in the air, and then crushed in a mortar. The pulverized powder was sized through a 200-mesh sieve to obtain cerium dioxide particles containing auxiliary components.

Subsequently, iron nitrate (Fe (NO ₃ ) ₃ · 9H ₂ O) powder, manganese nitrate (Mn (NO ₃ ) ₂ · 6H ₂ O) powder and water were weighed, and these powders were weighed in water in the container. To obtain an aqueous solution containing iron (Fe) and manganese (Mn). The mixing ratio (molar ratio) of iron and manganese is as shown in “Fe / Mn molar ratio” in Table 1. The cerium dioxide particles containing the auxiliary component were weighed and mixed with the aqueous solution, and the aqueous solution was stirred at 90 ° C. for about 5 hours using a hot stirrer. After visually confirming that the moisture in the container had disappeared, the mixture in the container was sufficiently dried at 90 ° C. for about 5 hours in a dryer to obtain a powder containing iron, manganese and cerium dioxide particles. The dried powder was fired at 700 ° C. in the air, and then crushed in a mortar. The pulverized powder was sized through a 200-mesh sieve to obtain cerium dioxide particles retaining iron and manganese oxides (metal oxides). Thus, the oxidation catalyst of Examples 1-11 was produced.

  Table 2 shows the molar ratios of the constituent components in the oxidation catalysts of Examples 1 to 11 (and Comparative Examples 1 to 3). In Table 2, the value obtained by converting the molar ratio of each constituent component into the mass ratio, and the mass ratio in terms of oxide of the auxiliary component with respect to cerium contained in the cerium dioxide particles (that is, the mass ratio of the oxide of the auxiliary component) The value divided by the mass ratio of cerium dioxide) is also shown. In addition, all the mass ratios of the metal oxide with respect to the whole oxidation catalyst are 20 mass%.

  In producing the catalyst-supporting structure, first, powdered silicon carbide (SiC), a binder material, a pore former, a binder, and water were mixed to prepare a molding material. In Examples 1 to 8, metal silicon was used as a binder material, and in Examples 9 to 11, a cordierite forming material was used as a binder material. The cordierite-forming raw material is a raw material from which cordierite crystals are generated by firing. A kneaded material was kneaded to form a kneaded material, and the kneaded material was extruded to obtain a honeycomb-shaped formed body. The formed body was dried and fired to obtain a honeycomb structure.

  In Table 1, in the column of the material of the base material, the honeycomb structures of Examples 1 to 8 are indicated as “Si-bonded SiC”, and the honeycomb structures of Examples 9 to 11 are indicated as “Cordierite”. The open porosity of the honeycomb structures of Examples 1 to 8 was 41.0%, and the average pore diameter was 11.0 μm. The open porosity in the honeycomb structures of Examples 9 to 11 was 58.0%, and the average pore diameter was 13.0 μm. The open porosity was measured using the Archimedes method, and the average pore diameter was measured using the mercury intrusion method (based on JIS R1655).

  Subsequently, the powder of the oxidation catalyst and water were weighed, and the powder was mixed with water in a container to obtain a slurry. The entire honeycomb structure was immersed in the slurry, and after a while, the honeycomb structure was taken out. Air was blown onto the honeycomb structure using an air gun to remove the slurry adhering to the outer surface of the honeycomb structure. Thereafter, the honeycomb structure was sufficiently dried at 90 ° C. for about 2 hours in a dryer, and the weight of the honeycomb structure was measured. Until the weight of the honeycomb structure after drying increases from the pre-measured weight before slurry addition by an amount corresponding to the supported amount of oxidation catalyst shown in “Catalyst loading” in Table 1, , Drying of the honeycomb structure, and weight measurement of the honeycomb structure were repeated. Thereafter, the honeycomb structure was fired at 300 ° C. In this manner, the oxidation catalyst was supported in the pores of the partition walls of the honeycomb structure, and catalyst supporting structures of Examples 1 to 11 were obtained.

(Comparative Examples 1-3)
Production of the catalyst-supporting structures of Comparative Examples 1 to 3 is the same as that of Examples 1 to 11 except that auxiliary components (lanthanum, aluminum, and iron) were not used in the production of the oxidation catalyst. In Comparative Examples 1 to 3, metal silicon was used as a binder raw material when the honeycomb structure was manufactured.

(Identification of constituent crystal phase of oxidation catalyst)
The constituent crystal phases in the prepared oxidation catalyst were identified. In identifying the constituent crystal phases, first, an X-ray diffraction pattern was obtained using an X-ray diffractometer. As the X-ray diffractometer, a rotating counter-cathode X-ray diffractometer (RINT, manufactured by Rigaku Corporation) was used. The conditions for the X-ray diffraction measurement were a CuKα radiation source, 50 kV, 300 mA, and 2θ = 10 to 60 °. The analysis of the X-ray diffraction data was performed using “X-ray data analysis software JADE7” manufactured by MDI, and the constituent crystal phase of the oxidation catalyst was identified. Table 3 shows the identification results of the constituent crystal phases for the oxidation catalysts of Examples 1 to 11 and Comparative Examples 1 to 3. Table 3 shows the identification results of the constituent crystal phases before and after the heat treatment described later. In Table 3, “◯” written in each crystal phase column means the presence of the crystal phase, and “-” means the absence of the crystal phase. In Examples 1 to 4, 7 to 11 and Comparative Examples 1 to 3, the presence of FeMnO ₃ was confirmed after the heat treatment.

(Heat resistance test)
As a heat resistance test, the NO ₂ conversion rate before and after the heat treatment was measured in each catalyst supporting structure. Table 4 shows the results of heat resistance tests on the catalyst-supporting structures of Examples 1 to 11 and Comparative Examples 1 to 3. Table 4 also shows the measurement results of the initial pressure loss (initial pressure loss) in the gas at a constant flow rate for each catalyst supporting structure before the heat treatment.

In the heat treatment, a mixed gas of 10% oxygen (O ₂ ), 10% water vapor (H ₂ O), and 80% nitrogen (N ₂ ) is heated to 750 ° C., and the catalyst-supporting structure is heated in the mixed gas for 16 hours. Retained. In the measurement of the NO ₂ conversion rate, each catalyst supporting structure was processed into a sample piece having a diameter of 25.4 mm and a length of 50.8 mm, and the outer peripheral surface of the sample piece was coated. Using this as a measurement sample, an automobile exhaust gas analyzer (manufactured by Horiba, Ltd., SIGU1000) was used for evaluation. Specifically, the measurement sample was set in a reaction tube in a temperature raising furnace and maintained at 250 ° C. In addition, a mixed gas of nitrogen monoxide (NO) 200 ppm, oxygen 10%, and the remainder nitrogen was heated to 250 ° C. and introduced into the measurement sample in the reaction tube. The measurement gas measuring device gas (the exhaust gas) discharged from the sample (Horiba Ltd., MEXA-6000FT) analyzes were used to obtain NO concentration and NO ₂ concentration in the exhaust gas. The NO ₂ conversion rate was determined by ((b / (a + b))) where the NO concentration was a and the NO ₂ concentration was b.

Table 4 also shows the deterioration rate of the NO ₂ conversion rate. Deterioration rate of NO ₂ conversion rate, the NO ₂ conversion rate before the heat treatment c, and NO ₂ conversion rate after the heat treatment as d, was determined by ((c-d) / c ). In Table 4, an item of “total evaluation” is further provided. For comprehensive evaluation, the catalyst-supporting structure having a NO ₂ conversion rate after heat treatment of 15% or more and an initial pressure loss of 2.15 kPa or less is marked with “◯”, and the NO ₂ conversion rate after heat treatment Is less than 15%, or “x” is attached to the catalyst-supporting structure in which the initial pressure loss exceeds 2.15 kPa.

As shown in Table 4, in the catalyst support structures of Examples 1 to 11 in which the cerium dioxide particles contain auxiliary components (lanthanum, aluminum and iron), the catalyst support of Comparative Examples 1 to 3 in which the cerium dioxide particles do not contain the auxiliary components Compared to the structure, the deterioration rate of the NO ₂ conversion rate was low. Therefore, it turns out that heat resistance improves in an oxidation catalyst because a cerium dioxide particle contains an auxiliary component. Comparative Example 2 and Examples 1, 3, 4, 7, 8 and Comparative Example 3 in which the type of base material, the amount of catalyst supported, and the mixing ratio of iron and manganese (Fe / Mn molar ratio) are the same. When comparing Example 2 with Example 2, the catalyst support structure of Example had a lower initial pressure loss than the catalyst support structure of Comparative Example. Furthermore, the NO ₂ conversion rate after the heat treatment also increased due to a decrease in the deterioration rate of the NO ₂ conversion rate.

<Modification>
Various modifications can be made to the oxidation catalyst 2, the catalyst support structure 1, the method for manufacturing the oxidation catalyst, and the method for manufacturing the catalyst support structure.

  In the cell structure carrying the oxidation catalyst 2, various shapes may be adopted as long as the inside is partitioned into a plurality of cells by partition walls. In the oxidation catalyst 2, the cerium dioxide particles 21 may hold other substances in addition to the metal oxide 22.

  The method for producing the oxidation catalyst and the method for producing the catalyst supporting structure are not limited to those described above, and may be variously changed. The oxidation catalyst 2 may be used for various purposes other than the filter.

  The configurations in the above-described embodiments and modifications may be combined as appropriate as long as they do not contradict each other.

DESCRIPTION OF SYMBOLS 1 Catalyst support structure 2 Oxidation catalyst 10 Honeycomb structure 12 Partition 13 Cell 21 Cerium dioxide particle 22 Metal oxide S11, S12, S21, S22 Step

Claims (8)

An oxidation catalyst,
Cerium dioxide particles containing an auxiliary component that is at least one of lanthanum, aluminum and iron;
A metal oxide containing iron and manganese and held in the cerium dioxide particles;
An oxidation catalyst comprising: The oxidation catalyst according to claim 1,
The oxidation catalyst, wherein the mass ratio of the auxiliary component is 3 to 45 mass% in terms of oxide with respect to cerium contained in the cerium dioxide particles. The oxidation catalyst according to claim 1 or 2,
The oxidation catalyst, wherein the mass ratio of the metal oxide is 5 to 40% by mass with respect to the whole oxidation catalyst. A catalyst carrying structure,
A cell structure that is internally partitioned into a plurality of cells by partition walls;
The oxidation catalyst according to any one of claims 1 to 3, supported on the partition wall;
A catalyst-supporting structure comprising: The catalyst-supporting structure according to claim 4,
A catalyst-supporting structure characterized by not containing a noble metal. A method for producing an oxidation catalyst, comprising:
Producing cerium dioxide particles comprising an auxiliary component that is at least one of lanthanum, aluminum and iron;
Holding the metal oxide containing iron and manganese in the cerium dioxide particles;
A process for producing an oxidation catalyst, comprising: A method for producing a catalyst-supporting structure, comprising:
A preparation step of preparing a cell structure whose interior is partitioned into a plurality of cells by a partition;
A supporting step of supporting the oxidation catalyst produced by the method for producing an oxidation catalyst according to claim 6 on the partition wall;
A process for producing a catalyst-supporting structure, comprising: It is a manufacturing method of the catalyst carrying structure according to claim 7,
In the supporting step, a liquid in which the oxidation catalyst is dispersed is applied to the cell structure.

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